Gas turbine engines are being made more efficient as demand for high performance engines continues. Obtaining the best possible performance requires that the system have the ability to maintain adequate exhaust cooling while maximizing the momentum recovery of fan air, for thrust. Doing this is extremely important for achieving increased range while permitting efficient augmentation of the engine output when needed without heat damage to the engine case inner liner.
These engines operate in two modes, in some cases with both modes, in which, under augmentation, a fan stream is used to cool the engine fan case, and when the fan stream and the core stream are mixed with good pressure recovery. During thrust augmentation, fuel is added downstream of the engine itself in an augmentor or afterburner, which consumes a large amount of fuel and therefore is only used when a sudden boost in thrust is required.